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Abstract
In northeastern Portugal, it has been difficult to identify the threshold value of exchangeable magnesium (Mg) below which Mg deficiency occurs in chestnuts (Castanea sativa Mill.), since discrepancies have been observed in some areas, apparently associated with parent material. A regional survey was carried out in chestnut groves established in soils derived from bedrock of several geological formations, some of them rich in manganese (Mn). Leaf sampling and soil analysis were performed in 38 groves. The lowest value of foliar Mg concentration under good growing conditions was 1.8 g kg−1 and the highest value in trees with Mg deficiency symptoms was 1.5 g kg−1. Under acidic soil conditions higher levels of extractable Mn propitiate Mg deficiency. The soils need to reach higher exchangeable Mg (≥0.34 cmolc kg−1) in order to supply Mg. For lower values of extractable Mn, the deficiency appears when exchangeable Mg is <0.20 cmolc kg−1 of soil.
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ACKNOWLEDGMENTS
Field assistance was provided by José Rego. Chemical analyse was carried out in the Soil and Plant Laboratory of University of Trás-os-Montes and Alto Douro. This study was supported by grants from CITAB.